So far as the offence punishable under Section 149 of IPC making a member of unlawful assembly guilty of offence committed in prosecution of common object is concerned, three ingredients are required to be satisfied -

(1) there must be an unlawful assembly of five or more persons,

(2) there must be commission of offence by any member of an unlawful assembly, and

(3) such offence must be committed in prosecution of the common object of that assembly, or such as the members of that assembly knew to be likely to be committed in prosecution of that object.

The common object has to be inferred from the facts and circumstances of the case. It is enough if each has the same object in view and their number is five or more and they act as an assembly to achieve that object.

The common object may be formed at any stage by all or a few members of the assembly and the other members may join or adopt it. Once the ingredients of the offence punishable under Section 149 of IPC are established, it is immaterial that any person of unlawful assembly did nothing with his own hands. It is the common object which makes the person vicariously liable for the acts of an unlawful assembly.

Section 120-A of IPC defines 'criminal conspiracy'. It states that when two or more persons agree to do, or cause to be done-

(1) an illegal act, or

(2) an act which is not illegal by illegal means, such an agreement is designated a criminal conspiracy.

The proviso clarifies that no agreement except an agreement to commit an offence shall amount to a criminal conspiracy unless some act besides the agreement is done by one or more parties to such agreement in pursuance thereof. The explanation clarifies that it is immaterial whether the illegal act is the ultimate object of such agreement, or is merely incidental to that object.

Section 120-B of IPC, dealing with the punishment of criminal conspiracy, stipulates in sub-section (1) that whoever is a party to a criminal conspiracy to commit an offence punishable with death imprisonment for life or rigorous imprisonment for a period of two years or upwards, shall, where no express provision is made within Code for the punishment of such a conspiracy, be punished in the same manner as if he had abetted such offence.

In the decision of the Apex Court in the case of Mukesh v. State (NCT of Delhi), reported in (2017) 6 SCC 1, the purpose of the provisions and principles are highlighted.

The underlying purpose for the insertion of Sections 120-A and 120-B of IPC was to make a mere agreement to do an illegal act or an act which is not illegal by illegal means punishable under law.

Criminal thoughts in the mind when take concrete shape of an agreement to do or cause to be done an illegal act or an act which is not illegal by illegal means then even if nothing further is done an agreement is designated as a criminal conspiracy.

The decision in Mukesh's case lays down that it is extremely rare that direct evidence in proof of conspiracy can be forthcoming from wholly disinterested quarters or from utter strangers. Hence, the criminal conspiracy can be proved by circumstantial evidence.

The illegal act may or may not be done in pursuance of agreement, but the very agreement is an offence and is punishable. Entering into an agreement by two or more persons to do an illegal act or legal act by illegals means is the very quintessence of the offence of conspiracy.

While dealing with the act of criminal conspiracy, the Court has to enquire whether the two persons are independently pursuing the same end or they have come together in the pursuit of an unlawful object. The former does not render them conspirators, but the latter does.

It is essential that an offence of conspiracy requires some kind of physical manifestation of agreement. The evidence as to the transmission of thoughts sharing the unlawful design may be sufficient.

The decision in Mukesh's case further holds that to constitute a criminal conspiracy, it is not necessary that all the conspirators must know each and every detail of the conspiracy. Neither is it necessary that every one of the conspirators takes active part in the commission of each and every conspiratorial acts.

The conspiracy is seldom an open affair and agreement can be inferred by necessary implication. Everything said, written or done by any of the conspirators in execution or furtherance of the common purpose is deemed to have been said, done, or written by each of them. And this joint responsibility extends not only to what is done by any of the conspirators pursuant to the original agreement, but also to collateral acts incident to and growing out of the original purpose.